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Years ago in high school we turned a friends old Honda into something that looked like the Millennium Falcon inside.
We made some thin boxes out of 1/8" plywood that fit into different places in the car (dash, ceiling, console, etc.), covered the faces with thin sheet metal then drilled holes for switches and LEDs in small grids and lines with random tech sounding labels (Flux Capacitor, Alpha Charge, Impulse Power, Shields, etc.) Then we wired them up essentially at random so flipping a switch would turn on/off a few random lights. Being copilot in that car was awesome. I think we Velcored them in place so we didn't completely destroy the interior.

I dunno about the 'coolest' thing you can do, but you could install an interrupt switch between your ignition and your starter. The idea is that you hide the switch somewhere in your car (like the ashtray) and simply flip the switch to 'off' when you leave your car. Without knowing where the switch is (or even that it's there!) a car thief may try to jam your ignition and force the car to start, but without toggling the interrupt switch the car will not start.

unfortunately a more savvy thief will find another way to start your car with or without the toggle :(